The exhaust purifying device of a general-purpose engine may include an exhaust passage provided with a catalytic converter, a secondary air passage having a downstream end connected to an intermediate point of the exhaust passage and an upstream end communicating with the atmosphere and a reed valve provided in an upstream end of the secondary air passage to permit air flow from the atmosphere to the exhaust passage but not in the opposite direction. See JP2014-173518A, for instance. The reed valve is configured to close the secondary air passage during the positive pressure phase of the exhaust pulsation that is produced in the exhaust passage, and open the secondary air passage during the negative pressure phase of the exhaust pulsation. Owing to this action of the reed valve, secondary air (fresh air) can be supplied to the catalytic converter while the exhaust is prevented from being released via the secondary air passage. The secondary air may also be introduced into the exhaust passage by using a pump, but the use of a reed valve is more advantageous in terms of cost and size.
However, when a reed valve is used for introducing the secondary air into the exhaust passage, because the open period of the reed valve is dictated by the negative pressure that may be created by the exhaust pulsation, the quantity of the secondary air that may be supplied to the catalytic converter may not be so great as desired. To overcome this problem, it was proposed to adjust the length of the secondary air passage so that the negative pressure that is created in the reed valve may be amplified by the pulsation of air in the secondary air passage, and the quantity of the secondary air admitted by the reed valve may be increased. See JP56-127313U, for instance.
Therefore, there has been a demand to increase the quantity of the secondary air that can be supplied to the catalytic converter of an exhaust system of internal combustion engines without complicating the exhaust system.